Various kinds of motor are used as driving sources for a lens moving mechanism for a zooming lens, a focusing lens, and the like in an optical devices such as a camera, a moving mechanism such as a diaphragm mechanism for brightness adjustment, and a moving mechanism provided in electrical products. For example, Japanese Patent No. 2559827 discloses an optical device using a pulse motor as a driving source for moving a movable lens or a diaphragm device for brightness adjustment.
In recent years, effort has been made on size reduction, less power consumption and high performance for the optical devices and electrical products. Thus, the motor used for a driving source has been required not only improved performance, for example, the starting performance, torque, operation stability, and energy efficiency but also size reduction.
For example, a motor for an optical system, focusing or zooming may be arranged around the periphery of a lens in a lens barrel. Thus, it is required for the motor to be arranged in a limited space. Moreover, according to the recent requirements for size reduction and improved performance, size reduction of the barrel section and proper speed and accuracy for zooming and focusing are required, i.e. balance between driving performance such as output power and the installation space is required for the motor. Such tendency is almost same in the moving mechanism provided in electrical products also, i.e. compact moving mechanism and high performance for driving source is required to realize both size reduction and high performance.
To catch up with such requirements, Japanese Patent Laid Open No. 2005-57903 discloses a technology on a step motor arranged in a barrel ground plate provided with a cylindrical lens or a brightness adjustment device in which output power is increased without increasing the outer diameter. The motor disclosed in Japanese Patent Laid Open No. 2005-57903 adopts a stator having a special shape and provided with a rotatable rotor provided with an annular magnet, two coils and four outer magnetic pole sections arranged around the outer periphery of and close to the magnet, and two inner magnetic pole sections arranged on the inner periphery of the magnet. In addition, arrangement of the outer magnetic pole sections is adjusted according to the angle of each outer magnetic pole section set against to rotating center of the rotor.
Because of improvement of performance in optical system devices in recent years, further improved speed and accuracy for zooming and focusing has been required. However, the pulse motors (step motors) disclosed in Japanese Patent No. 2559827 and Japanese Patent Application No. 2005-57903 which provides a starting torque lower than the torque in a constant speed motion may fail quick start when high load is required. In addition, an open loop-type system for motor control hardly follows a speed deviation caused by a variation of torque loaded.
Thus, to satisfy the above-described requirements, a three-phase brushless DC motor may be a solution to improve performance as a driving source. A three-phase brushless DC motor has a high starting torque, high acceleration performance, and excellent in speed control, i.e. it is suitable as a driving source for moving mechanisms that require quick and delicate operation.
However, as improvement in driving performance requires increased external size in the conventional three-phase brushless DC motor, an increase in the output power of the three-phase brushless DC motor installed in a limited installation space has been a challenge. For example, in an imaging device, a plurality of lenses arranged in a lens barrel are made move in the direction of an optical axis for zooming and focusing. In an imaging device with an auto-focusing function or the like, the driving force of a three-phase brushless DC motor is used to move these lenses in the direction of the optical axis. In this case, as the motor should be arranged inside the lens barrel, the three-phase brushless DC motor is arranged in an annular space in the lens barrel or the like as shown in FIG. 6. The installation space of the motor will be described taking this case as an example.
FIGS. 6(1) to 6(3) are schematic diagrams showing a conventional example in which a motor is mounted on a ground plate that is an annular flat plate on which a lens can be installed in center of a lens barrel. FIG. 6(1) shows an example in which a brush DC motor 100 is arranged in the lens barrel. In the moving mechanism, a speed reducer is provided to transmit the rotation of the motor as a driving source. However, a speed reduction ratio is required to be made big because torque is low when the brush DC motor 100 is used. As a result, a loud driving sound and a low transmission speed not suitable for quick response may arise as a problem to be solved.
FIG. 6(2) shows an example in which a conventional three-phase brushless DC motor 101 is provided. When the conventional three-phase brushless DC motor is used, the external size of the motor is restricted by the size of an annular piece in the direction of an arrow (a) as shown in FIG. 6(2). Thus, the diameter of a stator needs to be smaller than the size of an annular piece in the direction of the arrow (a). In this case, the diameter of a rotor should be reduced to make achieving of big torque hard.
In contrast, the torque can be increased by increasing the size of a rotor 200 in a three-phase brushless DC motor 102 as shown in FIG. 6(3). However, the volume for wound coil should be increased against to the size of the rotor 200 and it may result increased external size in the three-phase brushless DC motor 102. So, it has been difficult to provide a high-torque three-phase brushless DC motor as a motor arranged in a limited space, for example, in the lens barrel.
In view of the above-described problems, an object of the present invention is to provide a three-phase brushless DC motor having a high torque and stable operations with reduced installation space.